Surfactant Protein D (SP-D) plays important roles in the defense against inhaled microorganisms, contributes to the pulmonary response to antigenic challenge, and participates in the regulation of surfactant lipid homeostasis. SP-D can directly interact with neutrophils, and can modulate the anti-viral, anti-bacterial, and anti-fungal functions of neutrophils in vitro. However, there is also evidence that neutrophils contribute to the degradation and clearance of SP-D in the setting of acute lung injury, suggesting a complex interplay between neutrophils and SP-D in vivo. We have recently observed that SP-D is specifically degraded by the three major human neutrophil serine proteases: neutrophil elastase (NE), cathepsin G (CG), and proteinase 3 (PR3) with the liberation of similar, high molecular weight, disulfide-crosslinked fragments; similar cleavage is mediated by murine neutrophils and whole neutrophil lysates. Given the above, we hypothesize the neutrophil-derived proteases specifically degrade SP-D within the functionally important lectin domains. We further hypothesize the neutrophil serine proteases contribute to the enhanced clearance of SP-D in the setting of LPS challenge, thereby contributing to a depletion of functional forms of this host defense protein in the interval prior to compensatory increases in SP-D production. Accordingly, we propose: 1) to characterize the effects of purified neutrophil-derived serine proteases on SP-D structure and biological activity; 2) to examine mechanisms of neutrophil-mediated proteolysis in vitro with emphasis on the potential roles of "quantal proteolysis" and membrane-associated serine proteases; and 3) to examine the potential roles of neutrophil proteases on SP-D degradation and clearance in vivo. The contributions of serine proteases will be assessed in vitro using protease-deficient murine neutrophils, and the potential roles of these enzymes in SP-D clearance and degradation will be examined in vivo using murine models of protease deficiency in combination with models of acute lung injury. Together, these studies should provide important new information relating to the mechanisms that could determine the amount and functional activity of SP-D in the setting acute lung injury.